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XmlNamespaceDeclarationsAttribute Class

Specifies that the target property, parameter, return value, or class member contains prefixes associated with namespaces that are used within an XML document.


Namespace:  System.Xml.Serialization
Assembly:  System.Xml (in System.Xml.dll)

[AttributeUsageAttribute(AttributeTargets.Field|AttributeTargets.Parameter|AttributeTargets.Property|AttributeTargets.ReturnValue, AllowMultiple = false)]
public class XmlNamespaceDeclarationsAttribute : Attribute

The XmlNamespaceDeclarationsAttribute type exposes the following members.

Public methodXmlNamespaceDeclarationsAttributeInitializes a new instance of the XmlNamespaceDeclarationsAttribute class.

Public methodEqualsInfrastructure. Returns a value that indicates whether this instance is equal to a specified object. (Inherited from Attribute.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeReturns the hash code for this instance. (Inherited from Attribute.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodMatchWhen overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from Attribute.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)

The XmlNamespaceDeclarationsAttribute attribute can be applied only once in a class to a field or property that returns an XmlSerializerNamespaces object.

The XmlNamespaceDeclarationsAttribute allows you to store the prefixes and the associated namespaces used in an XML document. For example, one common usage of the attribute is to store XPath data, as it is defined by the World Wide Web Consortium ( document, XML Language (XPath) Version 1.0. In brief, an XPath is a string that contains many namespace prefixes and local names, along with some other syntax.

The XPath language allows for the association of a prefix with a path and using the prefix within the XML document. For example, the following XML document named "select" contains a prefix ("cal") associated with a specific URI ( The element contains an attribute named "path" that contains the XPath.

 <select xmlns:cal ="" path="cal:appointments/@startTime" />

The schema for this might look like the following.

 <element name="select">
          <attribute name="path" />

Without the XmlNamespaceDeclarationsAttribute, the association between the prefix and the namespace is lost.

To retain the association between the prefix and the namespace URI, add a member that returns an XmlSerializerNamespaces object and apply the XmlNamespaceDeclarationsAttribute attribute to the member, as shown in the following C# and Visual Basic code.

 // C#
 public class Select {
   [XmlAttribute] public string path;
   [XmlNamespaceDeclarations] public XmlSerializerNamespaces xmlns;

 ' Visual Basic
 Public Class Select
    <XmlAttribute> Public path As String
    <XmlNamespaceDeclarations> Public xmlns As XmlSerializerNamespaces
 End Class

When serialized, the schema for the generated XML document contains the XML Schema definition (XSD) element named appinfo. The element further contains a metadata element named keepNamespaceDeclarations, set to the name of the member that contains the namespace declarations. The following XML fragment shows the schema.

 <xs:element name="select">
          <xs:attribute name="path" />

On deserialization, the xmlns field contains an XmlSerializerNamespaces object that contains all namespace prefix definitions.

On serialization, the user can add prefix-namespace pairs to the XmlSerializerNamespaces object using the Add method. This is shown in the following C# and Visual Basic code.

 // C#
 using System;
 using System.IO;
 using System.Xml.Serialization;
 public class Select {
    public string xpath;
    public XmlSerializerNamespaces xmlns;
 public class Test {
    public static void Main(string[] args) {
       Select mySelect = new Select();
       mySelect.xpath = "myNS:ref/@common:y";
       mySelect.xmlns = new XmlSerializerNamespaces();
       mySelect.xmlns.Add("MyNS", "");
       mySelect.xmlns.Add("common", "");
       XmlSerializer ser = new XmlSerializer(typeof(Select));
       ser.Serialize(Console.Out, mySelect);
 // Output:
 // <?xml version="1.0" encoding="IBM437"?>
 // <select xmlns:xsd="" xmlns:xsi="" 
 // xmlns:common="" xmlns:MyNS="" xpath="myNS:ref/@common:y" />
 ' Visual Basic
 Imports System
 Imports System.IO
 Imports System.Xml.Serialization
 <XmlRoot("select")> _
 Public Class SelectPath
    <XmlAttribute> _
    Public xpath As String 
    <XmlNamespaceDeclarations> _
    public xmlns As XmlSerializerNamespaces 
 End Class
 Public Class Test 
    Public Shared Sub Main() 
       Dim mySelect As SelectPath = New SelectPath()
       mySelect.xpath = "myNS:ref/@common:y"
       mySelect.xmlns = New XmlSerializerNamespaces()
       mySelect.xmlns.Add("MyNS", "")
       mySelect.xmlns.Add("common", "")
       Dim ser As XmlSerializer = New XmlSerializer(mySelect.GetType)
       ser.Serialize(Console.Out, mySelect)
    End Sub
 End Class
 ' <?xml version="1.0" encoding="IBM437"?>
 ' <select xmlns:xsd="" xmlns:xsi="" 
 ' xmlns:common="" xmlns:MyNS="" xpath="myNS:ref/@common:y" />

Also note that the member to which the attribute is applied contains only the prefix-namespace pairs that belong to the XML element defined by the class. For example, in the following XML document, only the prefix pair "cal" is captured, but not the "x" prefix. To get that data, add a member with the XmlNamespaceDeclarationsAttribute to the class that represents the root element.

 <?xml version="1.0"?>
 <x:root xmlns:x="">
   <x:select xmlns:cal="" path="cal:appointments/@cal:startTime" />

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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